OED2 adds (whimsically??)

3. attrib., quasi-adj. Stupidly conservative.
1680 R. Bolron Narrative 9 The Jesuits+are the most Zealous for the propagation of their Religion in their old Mumpsimus Way. 1821 Syd. Smith Wks. (1859) I. 330/2 All the fat and sleek people+the mumpsimus, and ‘well as we are’ people, are perfectly outrageous at being compelled to do their duty.